Traditionally explosives have been prepared on site with metered flow provided by various augers, vibrators and mixers feeding into pumps for delivery to blast holes in the material to be blasted. Uneven terrain, exposure to climate and lack of quality control testing has normally resulted in poor blasting results. Other methods using large concrete mixers require handling of extreme weights and are subject to differential segregation which provides poor repeatability in blasting. To improve on quality control, fixed plants using rigid quality control have been tried where the final product is shipped to the field in large containers and a pump is used to discharge the mixture for use. This system provides initially a better product but transportation over rough roads may result in segregation of the ingredients.
Aqueous slurry blasting compositions have been prepared in a variety of forms with a view to improving their reliability and stability in the field. U.S. Pat. No. 3,390,030 discloses an aqueous slurry blasting composition which employs the use of an aeration agent to lower the specific gravity of the composition and thereby improve its sensitivity. The aeration agent is in the form of a sodium nitrite or sodium bicarbonate which lowers the specific gravity of the aqueous slurry blasting agent at the time of blasting. The ammonium and sodium nitrates, as used in the blasting composition, are dissolved in a hot solution at approximately 140.degree. F., thus requiring the preparation of the composition in a plant operation. The aqueous slurry also involves the use of ethylene glycol and guar gum as the thickener. The non-explosive carbonaceous fuel is ground coal, although it is suggested that other carbonaceous fuels such as glycols, amine nitrates and granulated sugar may be used.
The use of aluminum as a sensitizer agent in ammonium nitrate explosives has become quite popular, as also exemplified in this patent. The flaked aluminum entraps air on its surfaces assisting in inclusion of tiny air bubbles in the slurry. Such bubbles in an explosive serve to provide "hot spots" which help initiate the compositions.
Canadian patent No. 804,540 discloses an aerated aqueous slurry explosive composition using an inorganic oxidizing salt such as ammonium nitrate. Incorporating approximately 1% by weight of surfactant provides air entrainment within the composition. The air entrainment efficiency of the composition significantly improves the sensitivity of the composition to detonation by the high explosive pentaerythritol tetranitrate (PETN) boosters or the like. The slurry composition includes ammonium and sodium nitrate as the inorganic oxidizing agent. Pine oil, ethylene glycol and guar gum are used as thickening agents in combination with sugar and flaked aluminum.
Many other forms of explosive compositions have been prepared and tried as exemplified in Canadian patent No. 610,246 which is directed to a basic ammonium nitrate explosive composition, where carbon is used as the sensitizer for the inorganic oxidizing agent. Canadian patent No. 657,242 discloses the use of a heated solution in combining the ammonium nitrate into the explosive composition. Canadian patent No. 657,934 requires the use of a finely divided ammonium nitrate with a liquifiable carbon such as fuel oil. Canadian patent No. 713,491 uses a surfactant in combination with nitric acid where the fuel material is essentially unreactive with the nitric acid component to provide a stable composition. Canadian patent No. 829,230 discloses the use of hexamethylene triamine sensitizers for an ammonium nitrate explosive composition.
With the explosive compositions disclosed in these patents, no attention has been directed to the mixing technique and as experience in the marketplace, these compositions are routinely mixed in various forms of rotating blade mixers and stirrers which are operated at a constant speed of mixing.
The method, apparatus and aqueous slurry explosive mixture in accordance with this invention provides a composition which has improved performance, is economical to manufacture and the apparatus is particularly compact lending itself for use as a mobile unit.